The present invention relates to an accessory for high voltage direct current (HVDC) energy cables, such as a cable joint or a cable termination.
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by an electrically conductive metal conductor sequentially covered by an inner polymeric layer having semiconducting properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconducting properties. Cables for transporting high voltage electric energy generally include at least one cable core surrounded by a screen layer, typically made of a metal or of a metal and a polymeric material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core or of a sheet longitudinally wrapped around the cable core.
The polymeric layers surrounding the conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during conventional use and with current overload.
The crosslinking process of polyolefin materials, particularly polyethylene, requires the addition to the polymeric material of a crosslinking agent, usually an organic peroxide, and subsequent heating at a temperature to cause peroxide cleavage and reaction. By-products are formed mainly deriving from the decomposition of the organic peroxide. In direct current (DC) energy cables operating with a continuous electrical field, such by-products, being entrapped within the crosslinked material, cause an accumulation of space charges which may generate electrical discharges and eventually insulation piercing. For instance, dicumyl peroxide, a commonly used crosslinking agent for cable insulation, gives place to methane (light by-product) and heavy by-products, mainly acetophenone and cumyl alcohol. Methane can be eliminated from the cable core with a short degassing process at a relatively low temperature (about 70° C.), while acetophenone and cumyl alcohol can be removed only by subjecting the cable core to a prolonged degassing process, at a temperature suitable to cause migration of the by-products (usually about 70° C.÷80° C.) and subsequent evaporation from the cable core. This degassing process must be performed for a long time (usually from 15 days to about 2 months, depending on the cable core dimensions) and is carried out batchwise in large degassing devices which can host a given cable length. This is particularly critical for HVDC cables.
Energy cable accessories are used in an energy network to restore the insulation and electric field control over a cable portion where the conductor was exposed, such as in case of connection between two energy cables or between an energy cable and another network component, such as a transformer, a generator, a bare conductor of an overhead line or the like. Cable accessories include cable joints and cable terminations.
A cable joint generally comprises a sleeve which is fitted over the conductors' connection. The sleeve may comprise different parts, each having a different function. Typically, starting from the innermost layer, a sleeve may comprise a tubular electrode made of a semiconducting material suitable for controlling the electrical field around the cable conductors' connection, a tubular insulating element which covers the electrode, and an outer semiconducting layer enveloping the tubular insulating element. The sleeve usually has a substantially cylindrical central portion and two substantially conical opposing end portions (stress-relief cones), typically made of semiconducting material.
A termination is a device fitted to the end of an energy cable to ensure its mechanical and electrical connection to a bare conductor and to maintain the insulation up to the point of connection; the bare conductor can be for example an overhead conductor or a different device, such as an electrical power source or a transformer, etc.
Terminations for high voltage electric cables typically comprise a sleeve made of elastomeric material and a conductive rod adapted to be connected to the bare conductor. A connector is provided inside the sleeve to mechanically and electrically connect the conductive rod to the electrical cable, so as to provide the electrical connection between the electrical cable and the bare conductor. The sleeve further comprises a semi-conductive conical insert (or stress-relief cone) for controlling the electrical field within the sleeve, in particular to avoid the presence of areas with high concentration of electrical field wherein undesired electrical discharges and breakdowns could occur. A conductive element is arranged in a radial outer position with respect to the semi-conductive cylindrical insert in a contact relationship therewith.
The insulating elements of pre-moulded cable joints or terminations are commonly made of crosslinked elastomeric ethylene copolymers, such as EPR or EPDM polymers. The accumulation of space charges in the matrix of an elastomeric polymer, such as an EPR or EPDM polymer, is a less relevant issue compared to XLPE polymers. These elastomeric polymers in fact generally contain charges dispersed in the polymeric matrix (e.g. lead oxides) which prevent the accumulation of space charges. For this reason, the accessories comprising insulating element made from the said elastomeric materials can be used—in principle—without being subjected to a degassing process for removing the crosslinking by-products.
However, since the accessories are to be used with HVDC cables, the accessories made of EPR/EPDM are precautionary subjected to degassing as well in order to minimize the risk of accumulation of space charges in the cable insulating layers. In the presence of intense currents, in fact, the crosslinking by-products embedded in the polymer matrix of the insulating element of the accessories can migrate towards the insulating layer of the degassed cables and consequently cause accumulation of space charges which eventually could result in the insulation piercing. It is apparent that the need for degassing the accessories substantially increases the time and production cost of the same.
It is therefore felt the need to provide pre-moulded accessories, particularly for HVDC cables, having at least an insulating element made from a crosslinked polymer material that can be used without being preventively degassed.
In US 2010/0314022 a process is described for producing an insulated DC cable with an extruded polymer based electrical insulation system, which comprises the steps of: providing a polymer based insulation system comprising a compounded polymer composition, preferably a compounded polyethylene composition; optionally cross-linking the polymer composition; and finally exposing the polymer based insulation system to a heat treatment procedure while the outer surface of the polymer based insulation system is covered by a cover impermeable to at least one substance present in the polymer based insulation system in a non-homogenous distribution, thereby equalizing the concentration of the at least one substance in the polymer based insulation system. The at least one substance comprises typically cross linking by-products and various additives, which typically increase the material conductivity. Preferably a thin metallic foil or similar is wrapped around the roll of DC cable. Alternatively, the impermeable cover can be the metallic screen or the outer covering or sheath arranged outside the metallic screen. The overall effect of such a process is that of equalizing the concentration of the crosslinking by-products within the insulating layer.
JP 64-024308 relates to a DC power cable provided with a space charge buffer layer placed between the inner semiconducting layer and the insulating layer or between the outer semiconducting layer and the insulating layer, the space charge buffer layer being formed by a copolymer of ethylene with an aromatic monomer, e.g. styrene, in an amount from 0.01 to 2 mol % per 1 mol of ethylene. Due to the resonance effect of the benzene ring of the aromatic monomer, the surrounding electron energy is absorbed and the formation of space charge is prevented, and in addition it is possible to improve the dielectric strength of the base polymer.
JP 02-253513 relates to a cross-linked polyethylene insulation cable that prevents oxidative degradation caused by contact with oxygen and enables continuous operation at high temperatures. As cracked residue of the organic peroxide, cumyl alcohol undergoes degradation to form α-methylstyrene and water. The deydradation of cumyl alcohol is accelerated in the presence of oxygen. The moisture that is formed may cause appearance of voids and bow-tie trees with consequent degradation of the insulating material. A plastic material containing an oxygen absorbent is arranged on the central part and the outer semiconducting layer of the conductor. As oxygen absorbent, a deoxidizer may be used, such as a commercially available product known as Ageless by Mitsubishi Gas Chemical Co., which is formed by iron oxide/potassium chloride.